1. Field of the Invention
The field of the invention concerns a method of extrapolating a two-dimensional grey-level image intended to be used in a radiography device, in particular of the tomosynthesis type.
2. Description of Related Art
In such a device, when a grey-level image is processed, it is necessary to “invent” information outside the field of the image (a field corresponding to the active surface of the detector of the radiography device). In particular, this is necessary when the pixels close to the edges of the detector are processed. When this extrapolation is carried out in an arbitrary fashion, artefacts and inconsistencies are created in the image processed. In the context of the application of digital tomosynthesis, such as digital tomosynthesis of the breast, the projection images are processed either during a pre-processing step (for example in order to apply a thickness compensation algorithm) or during a first filtering step in the context of a reconstruction algorithm of the filtered back-projection type. The edge-effect artefacts created during the pre-processing step propagate and are amplified in the reconstructed volume.
This is because, when an image must be thus processed, the lack of information at the limits of the field of the image often produces a few artefacts. The values of the pixels processed inside these areas are based on unknown data. Many approaches currently exist for “inventing” the information in these areas. Such approaches are described for example in the documents U.S. Pat. No. 6,307,909, EP 0 971 318, US 2007/0230652 which attempt to resolve this problem encountered in the context of tomography. Nevertheless, these methods are not applicable in the context of tomosynthesis of the breast because of the difference between the acquisition systems. Other more basic approaches such as replication of the last known value or replication by mirror exist. However, the information thus created is false in the context of an image to be processed composed of an object and a background: often the shape of the object is modified in the vicinity of the limits of the image. A second drawback may arise when consistency with the real data in terms of signal variation is not guaranteed and may therefore lead to over-estimations of intensity at the pixel being processed.
In an article by R M Lewitt entitled “Processing of incomplete measurement data in computed tomography” published in Medical Physics, volume 6, No. 5, September/October 1979, p 412-417, there is described a method of extrapolating this type of image using a three-dimensional mask of the object in order to evaluate the thickness of the object that the x-rays are passing through. However, in some applications, in particular in digital tomosynthesis of the breast, the 3D mask calculated is over-estimated because of the limited angulation described by the x-ray source in a radiography device for digital tomosynthesis of the breast. This over-estimation leads to a poor evaluation of the thickness of the object and then results in obtaining extrapolated images having inconsistencies and artefacts.